MIT engineers amplify signals from fluorescent sensors

CAMBRIDGE, MA – Fluorescent sensors, which can be used to label and image a wide variety of molecules, offer unique insights inside living cells. However, they can usually only be used in cells grown in a laboratory dish or in tissues close to the surface of the body, because their signal is lost when implanted too deeply.

MIT engineers have now found a way to overcome this limitation. Using a new photonics technique they developed to excite any fluorescent sensor, they were able to dramatically improve the fluorescent signal. With this approach, the researchers showed that they could implant sensors up to 5.5 centimeters deep in tissue and still get a strong signal.

This kind of technology could make it possible to use fluorescent sensors to track specific molecules inside the brain or other deep tissues of the body, for the purposes of medical diagnosis or monitoring the effects of drugs, according to the researchers.

“If you have a fluorescent sensor that can probe biochemical information in a cell culture or in thin layers of tissue, this technology allows you to translate all those fluorescent dyes and probes into thick tissue,” says researcher Volodymyr Koman at the MIT and one of the lead authors of the new study.

Naveed Bakh SM ’15, PhD ’20 is also a lead author of the paper, which appears today in Nature Nanotechnology. Michael Strano, Carbon P. Dubbs Professor of Chemical Engineering at MIT, is the lead author of the study.

Improved fluorescence

Scientists use many types of fluorescent sensors, including quantum dots, carbon nanotubes and fluorescent proteins, to label molecules inside cells. The fluorescence of these sensors can be seen by shining a laser light on them. However, this does not work in thick, dense tissue, or deep in tissue, because the tissue itself also emits fluorescent light. This light, called autofluorescence, drowns out the signal from the sensor.

“All tissues are autofluorescent, and that becomes a limiting factor,” says Koman. “As the sensor signal gets weaker and weaker, it is overtaken by tissue autofluorescence.”

To overcome this limitation, the MIT team found a way to modulate the frequency of the fluorescent light emitted by the sensor so that it could be more easily distinguished from tissue autofluorescence. Their technique, which they call Wavelength Induced Frequency Filtering (WIFF), uses three lasers to create a laser beam with an oscillating wavelength.

When this oscillating beam is projected onto the sensor, it causes the fluorescence emitted by the sensor to double in frequency. This makes it easy to distinguish the fluorescent signal from the background autofluorescence. Thanks to this system, the researchers were able to multiply by more than 50 the signal-to-noise ratio of the sensors.

A possible application for this type of detection is to monitor the effectiveness of chemotherapy drugs. To demonstrate this potential, the researchers focused on glioblastoma, an aggressive type of brain cancer. Patients with this type of cancer usually have surgery to remove as much of the tumor as possible and then receive the chemotherapy drug temozolomide (TMZ) to try to remove the remaining cancer cells.

This drug can have serious side effects, and it doesn’t work for all patients, so it would be helpful to have a way to easily monitor whether or not it’s working, Strano says.

“We’re working on technology to make small sensors that could be implanted near the tumor itself, which can give an indication of how much of the drug is getting to the tumor and if it’s being metabolized. You can place a sensor near the tumor and check from outside the body how effective the drug is in the real environment of the tumor,” he says.

When temozolomide enters the body, it breaks down into smaller compounds, including one called AIC. The MIT team designed a sensor that could detect AIC and showed they could implant it up to 5.5 centimeters into an animal’s brain. They were able to read the sensor signal even through the animal’s skull.

Such sensors could also be designed to detect molecular signatures of tumor cell death, such as reactive oxygen species.

“Any Wavelength”

In addition to detecting TMZ activity, the researchers demonstrated that they could use WIFF to enhance the signal from various other sensors, including carbon nanotube-based sensors that Strano’s lab previously developed to detect peroxide. hydrogen, riboflavin and ascorbic acid.

“The technique works at any wavelength and can be used for any fluorescent sensor,” says Strano. “Because you have so much more signal now, you can implant a sensor at depths in tissue that weren’t possible before.”

For this study, the researchers used three lasers together to create the oscillating laser beam, but in future work they hope to use a tunable laser to create the signal and further improve the technique. This should become more feasible as the price of tunable lasers decreases and they become faster, the researchers say.

To help make fluorescent sensors easier to use in human patients, researchers are working on sensors that are biologically resorbable, so they wouldn’t need to be surgically removed.

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